Generally, a display device is classified into a self-emissive type such as a plasma display panel displaying an image using a plasma discharge system and a non-emissive type displaying an image using a light source of a backlight lamp.
As the non-emissive type display device, a liquid crystal display (LCD) device can be exampled. The LCD includes an inverter for supplying alternating current power to a backlight lamp and a dimming circuit for continuously varying brightness of the backlight lamp.
Meanwhile, as a dimming method, an analog dimming method that varies an amplitude of the voltage by detecting current flowing in the backlight lamp and feed-backing the detected current in the form of voltage is well known.
However, the analog dimming method couldn't meet the certificate standard by the Swedish Confederation Of Professional Employee (TCO) “03” since its dimming ratio of maximum luminance to minimum luminance is approximately 2:1.
Recently, a burst mode type dimming (or pulse width modulation (PWM) dimming) that can realize the dimming by adjusting an on/off duty of a power supply switch according to a dimming frequency has been used.
The burst mode type dimming will now be described with reference to the accompanying drawing.
Referring first to FIG. 1, an LCD device includes an outer signal input unit 10 for receiving an image signal from an external side and pre-processing the image signal, a decoder 20 for decoding the image signal pre-processed by the outer signal input unit 10 using MPEG 2, a scaler (or a video processor) 30 for converting the image signal decoded by the decoder 20 into a format appropriate for a screen on which the image will be displayed, and an display unit 40 for displaying the image signal scaled by the scaler 30.
The LCD device further includes an inverter 50 for supplying driving alternating current power for controlling brightness of the display unit 40, a dimming control unit 60 for performing the dimming by controlling a switching of the alternating current power supply, and a control unit 70 for generally controlling the display of the image signal as well as the dimming.
The display unit 40 includes a display panel 42 for displaying the image signal scaled by the scaler 30, and lamps 41 and 43 supplying light to the display panel 42.
As shown in FIG. 2, the control unit 70 applies a dimming frequency 1/T of the burst mode to the dimming control unit 60. The inverter 50 receiving the generated dimming signal controls the on/off Ton/Toff of the alternating current power supply for driving the lamps 41 and 43.
According to the above-described LCD device, although the image signals inputted from the external side have different formats having different vertical synchronizing frequencies, only one dimming frequency set in the dimming control unit 60 is used. Accordingly, resonance may be generated between the vertical synchronizing frequency freq_v_sync and the dimming frequency freq_Dim to generate a wavy noise on a predetermined portion of the screen.
For example, when the inputted image signals having different formats have vertical synchronizing frequencies of 56 Hz, 60 Hz, 70 Hz, 72 Hz, and 75 Hz while the dimming frequency set at the dimming control unit 60 is fixed at 65 Hz, resonance may be generated between a frequency of the image signal and the dimming frequency.
The resonance may be more frequently generated when the image signal having frequency of 60 Hz or 70 Hz similar to the dimming frequency of 65 Hz is inputted.
The noise may be also generated by the resonance between the horizontal synchronizing frequency and the dimming frequency. FIG. 3 illustrates wavy noise generated on a display.